Person:

Martin, Scot

The chemical composition of secondary organic aerosol (SOA) particles, formed by the dark ozonolysis of α-pinene, was characterized by a high-resolution time-of-flight aerosol mass spectrometer. The experiments were conducted using a continuous-flow chamber, allowing the particle mass loading and chemical composition to be maintained for several days. The organic portion of the particle mass loading was varied from 0.5 to >140 μg/m3 by adjusting the concentration of reacted α-pinene from 0.9 to 91.1 ppbv. The mass spectra of the organic material changed with loading. For loadings below 5 μg/m3 the unit-mass-resolution m/z 44 (CO2+) signal intensity exceeded that of m/z 43 (predominantly C2H3O+), suggesting more oxygenated organic material at lower loadings. The composition varied more for lower loadings (0.5 to 15 μg/m3) compared to higher loadings (15 to >140 μg/m3). The high-resolution mass spectra showed that from >140 to 0.5 μg/m3 the mass percentage of fragments containing carbon and oxygen (CxHyOz+) monotonically increased from 48% to 54%. Correspondingly, the mass percentage of fragments representing CxHy+ decreased from 52% to 46%, and the atomic oxygen-to-carbon ratio increased from 0.29 to 0.45. The atomic ratios were accurately parameterized by a four-product basis set of decadal volatility (viz. 0.1, 1.0, 10, 100 μg/m3) employing products having empirical formulas of C1H1.32O0.48, C1H1.36O0.39, C1H1.57O0.24, and C1H1.76O0.14. These findings suggest considerable caution is warranted in the extrapolation of laboratory results that were obtained under conditions of relatively high loading (i.e., >15 μg/m3) to modeling applications relevant to the atmosphere, for which loadings of 0.1 to 20 μg/m3 are typical. For the lowest loadings, the particle mass spectra resembled observations reported in the literature for some atmospheric particles.

A new instrument, a 1 × 3 tandem differential mobility analyzer (1 × 3-TDMA), was deployed in June 2007 in the Southern Great Plains, Oklahoma, USA to study the phase of ambient particles. Its primary measurement, the irreversibility of the hygroscopic growth factor, is obtained by reversibly cycling relative humidity (RH) by ±8% and testing for irreversible changes in diameter. In 101 runs, efflorescence occurred 72% of the time for particles sampled at ambient RH. Deliquescence occurred in 13% of the runs. The more frequent occurrence of efflorescence compared to deliquescence was explained at least in part by the distribution of ambient RH, which had a median of 80% and quartiles of 65% and 93% RH. The deliquescence and efflorescence events were nearly exclusive from one another and could be separated by Min[RH Ambient, Inlet RH] <40% for deliquescence and Max[RH Ambient, Inlet RH] >50% for efflorescence. In outlook, the data set from the 1 × 3-TDMA regarding the phase and hence water content of ambient particles can be used for validating regional chemical transport models of particle phase.

An unmanned aerial vehicle 1 (UAV) equipped with miniature monitors was used to study the vertical profiles of PM2.5 (particulate matter [PM] with a ≤2.5-μm diameter) and black carbon (BC) in Macau, China, from the surface to 500 m above ground level (AGL). Twelve- and 11-day measurements were conducted during February and March 2018, respectively. In total, 46 flights were conducted between 05:00 and 06:00 AM Local Time (LT) during the measurement days. The average concentrations of PM2.5 and BC were significantly lower in March (40.1 ± 17.9 and 2.3 ± 2.0 μg m−3 , respectively) when the easterly winds dominantly prevailed compared with the concentrations in February (69.8 ± 35.7 and 3.6 ± 2.0 μg m−3, respectively) when the northerly winds typically occurred. In general, PM2.5 concentrations decreased with height. A vertical decrement of 0.2 μg m−3 was observed per 10 m. BC concentrations exhibited diverse vertical profiles with an overall vertical decrement of 0.1 μg m−3 per 10 m. Meteorological analyses including back-trajectory analysis and atmospheric stability categorization (i.e., Pasquill stability class) revealed that both advection and convection transports may have notable influences on the vertical profiles of PM pollutants. The near-surface accumulation of PM pollutants is positively associated with atmospheric stability. The height of the planetary boundary layer influences atmospheric dispersion. In our study, the concentration of PM pollutants above the boundary layer was lower than that below the layer, thus exhibiting a sigmoid profile in some cases. The air mass origin and vertical wind influences the vertical profiles of the PM pollutants, particularly when the atmosphere was neutral. We presented that primary emissions, such as those from nearby fishing vessels, lighting of firecrackers and fireworks during the Chinese New Year (CNY), and the takeoff of civil flights from a nearby airport, may affect the vertical profiles of the PM pollutants and CO in different ways. In particular, the lighting of firecrackers and fireworks on February 16 (first day of the CNY) resulted in the elevated concentrations of PM2.5 and BC within 150 m AGL and the pronounced accumulation of CO at approximately 400 m AGL. The takeoff of a civilian flight on February 10 may have resulted in a substantial increase in the PM2.5 concentrations from 80.8 (±2.1) μg m−3 at the ground level to 119.2 (±9.3) μg m−3 at a height of 330 m. Although the results are confined to a height of 500 m AGL, the current study provides a rich dataset for PM vertical distributions along with explanations of its possible causes rather than the more commonly investigated spatiotemporal variations by conducting ground-based measurements.